Method of choosing soils as CEB or pavement layers based on geological and environmental information and mode of treatment

During research on the geological, mineralogical, mechanical and geotechnical nature of thick layers of soils developed on the rocks of the lower zone of the southern slopes of Mount Bambouto (West Cameroon), a couple of observations were made. The mechanical behavior of soils as construction materials is linked to the nature of the parent rock and its environment of formation. Four different types of rock have been noted: trachybasalt, orthogneiss, anatexite, and biotite-hornblende granitoid. These rocks in a hot and humid mountain climate, exposed to the monsoon wind, have weathered to give soils with exploitable characteristics in compressed earth bricks (CEB) and pavement layers. This article proposes a method of choosing a soil for pavement layers and for the production of fired earth bricks / earth bricks treated or not with hydraulic binders, based on the nature of the bedrock and the climatic environment. Several formulation models have been defined. The performance, fairness, accuracy and adequacy of these models with real values were made by metric evaluation. So, it is: • Preliminary method for quickly and easily formulating masonry units in clay soil.• Preliminary method for quickly and easily formulating pavement layers in clay soil.• Simple and reliable method for choosing clay soils as construction materials from elementary geological information.


a b s t r a c t
During research on the geological, mineralogical, mechanical and geotechnical nature of thick layers of soils developed on the rocks of the lower zone of the southern slopes of Mount Bambouto (West Cameroon), a couple of observations were made.The mechanical behavior of soils as construction materials is linked to the nature of the parent rock and its environment of formation.Four different types of rock have been noted: trachybasalt, orthogneiss, anatexite, and biotite-hornblende granitoid.These rocks in a hot and humid mountain climate, exposed to the monsoon wind, have weathered to give soils with exploitable characteristics in compressed earth bricks (CEB) and pavement layers.This article proposes a method of choosing a soil for pavement layers and for the production of fired earth bricks / earth bricks treated or not with hydraulic binders, based on the nature of the bedrock and the climatic environment.Several formulation models have been defined.The performance, fairness, accuracy and adequacy of these models with real values were made by metric evaluation.So, it is: • Preliminary method for quickly and easily formulating masonry units in clay soil.
• Preliminary method for quickly and easily formulating pavement layers in clay soil.
• Simple and reliable method for choosing clay soils as construction materials from elementary geological information.

Method details
This paper proposes four charts ( Figs. [1][2][3][4] based on 4 patterns ( Tables 1-9 ) that allow a quick reading of the formulation of construction materials (treated or untreated earth bricks, pavement layers) according to the intended use of the structure and the environment of the structure.It also provides 36 correlations for determining the physical and mechanical characteristics (compressive strength, bending strength, water absorption rate) depending on the treatment adopted for the identified earth bricks.In addition, twelve correlations are presented to determine the CBR bearing index according to the dosage of binder or not, the number of days of immersion in water, or of air drying of the clay soil.All these correlations have a coefficient of determination between 0.91 and 1, except that (correlations 24 and 28) of the water absorption of CEB from lateritic clay soils developed on anatexite, depending on the lime dosage (R 2 = 0.6968) and that of the compressive strength of CEBs from lateritic clay soils developed on granitoid, depending on the cement dosage (R 2 = 0.8883) which are less than 1.

Choosing of lateritic soil origin for pavement layers or CEBs
How to choose the soil and treatment?1st) Beforehand, determine: the availability of binders, the targeted resistance of the building (for CEBs), the targeted load-bearing class (for pavement layers) and the targeted durability of the work.2nd) Review the literature on local geology, climate and topography.3rd) Determine the nature of the bedrock, climate and topography.4th) If the geological, climatic and topographic characteristics correspond to those of one of the patterns, note the pattern and continue.Otherwise stop.5th) In the chosen model and based on the information from point 1, choose treatment method.Note: Use pavement design guide as part of pavement layers [1] .6th) Choose the corresponding formula, table or figure and determine the components of the materials (treated or untreated, nature and dosage of the binder, type of CEB, type of pavement layer).

Geological context
For a better relationship between the quality of construction materials (mechanical and physical quality of soil product/local construction material) and the bedrock, a review of the geological and environmental context was carried out ( Table 1 ).The different elements considered consist of: geological formation (1 st ), age (2 nd ), topography (3 rd ), climate (4 th ), mineral and geochemical combination (5 th ).

Use of CEBs: the specification terms
The rate of absorption decreases with the increase of cement, lime or temperature.These values are used to determine the appearance, the designation and the masonry environment of the various CEBs according to the "NC 106-107: 2002-2006 [2] reference ARS 674-675: 1996 [3] "standard.Table 9 indicates the different names of CEB according to each geological environment.-CEB treated with cement, -fired CEB.

Proposed formulas for determining the mode of treatment of the clay soil for CEB based on their origin
In the formulas No. 1 to 36, it should be noted that: x ( Table 10 ) is the increase in the percentage of the hydraulic binder according to an arithmetic sequence of reason 2 %, of first term 0 %; or the increase in the firing temperature according to an arithmetic sequence of reason 50 °C, of first term 950 °C.

Table 10
Corresponding x value for each formula.Proposed formulas for determining the mode of treatment of the clay soil for pavement layers based on their origin Among the four types of soils, those developed on trachybasalt and anatexite are fairly representative of the clayey and lateritic nature of all the soils.For these reasons, these two soils were selected to perform geological-geotechnical correlations in terms of pavement layers.Soils developed on trachybasalt is in pattern I and of class A-2-7(2) of HRB classification; while soils developed on anatexite in pattern IV and of class A-7-5 (0-12).Table 6 indicates the bearing capacity of the soil according to these patterns.
The climatic environment as defined in Table 1 is that of a hot and humid climate.The technical guide [1] and experience estimate a maximum flooding time in the rainy season of 4 days.Thus, the samples compacted with modified Proctor parameters are immersed for four days, then they are directly punched or dried for between 1 and 10 days.This allows to know the impact of variations in temperatures and precipitations on the pavement layers in lateritic clay soils.
In the formulas No. 37 to 48, it should be noted that for a fixed value of number of curing days in air after 4 curing days in water of compacted soils, x ( Table 6 ) is the increase in the percentage of the hydraulic binder according to an arithmetic sequence of reason 2 %, of first term 0 %.

Lateritic clay soils developed on trachybasalt (Pattern I)
y = CBR at 95% of OPM

Geology-geotechnics correlation: graphical pattern
Soils, products of rock weathering, are quite variable in space, which makes it difficult to represent geometrically the mechanical properties of the soils in space.Thus, this representation requires integration of the geological factors governing their implementation in order to control their interpolation [ 4 ] and [5] .The type of weathering (chemical or mechanical), the tectonics (soft or fragile), the age of the rocks (Precambrian or recent formations), the climate (temperatures, precipitations), the geomorphology (orography, hydrography) must also be considered.Furthermore, it is important to assess the local feasibility of the model so that it can be used, as in the case of a development project.The geotechnical data ( Tables 6-8 ) obtained are illustrated in bar diagrams for different patterns ( Tables 1-5 ).This allows a rapid and simple reading of the geotechnical abilities of the soils developed on different rocks according to its local, natural context.These geotechnical abilities are grouped into aptitude for the production of treated or untreated CEBs and soil bearing capacity in pavement layers.The different CEB formulation patterns and pavement layers are shown in Figs.1-4 .

=
binder or temperature: 0 = untreated CEB, Cx = treated CEB at x% of cement, Lx = treated CEB at x% of lime, y °C = CEB fired at y °C, S-0 = Untreated soil for pavement layer, S-L -i-j or S-C-i-j: "Clay soil (S) treated with (i)% of lime (L) or cement (C) after (j-4) days air curing, and 4 immersion days .Ci o C-i, Li or C-i, T °C Lateritic clay soils developed on trachybasalt (pattern I)

Fig. 1 .
Fig. 1.Compressive strength of CEB for the different models.

Fig. 2 .
Fig. 2. Water absorption of CEB for the different models.

Fig. 3 .
Fig. 3. Bulk density of CEB for the different models.

Table 2
Pattern I: Statistics of soil oxides and minerals (in weight-%) developed on trachybasalt.

Table 3
Pattern II: Statistics of soil oxides and minerals (in weight-%) developed on biotite and hornblend granitoid.

Table 4 Pattern
III: Statistics of soil oxides and minerals (in weight-%) developed on orthogneiss.

Table 5
Pattern IV: Statistics of soil oxides and minerals (in weight-%) developed on anatexite.

Table 7
Characteristics of soil for pavement layers.

Table 8
Physical and mechanical properties of CEB of various patterns (I, II, III and IV).

Table 9
Designation of CEB according to their resistance.